Five days ago I became a volunteer for the first clinical trial in Britain investigating the safety of a vaccine for the human form of bird flu. At the Churchill Hospital in Oxford, I went through the long process of enrolment, health checks, questions and blood tests before the research nurse was finally able to administer a shot of the yellow vaccine into my arm.

'Are you really sure you want to go through with this?', my husband asked as we sat in the kitchen the evening before the visit. 'What about the side effects. I mean, just how risky is this?' His apprehension was understandable, given the adverse publicity for the catastrophic Northwick Park trial in London, when six young men nearly died after receiving a previously untested compound.

Every day I wait for the side effects that might accompany the inoculation. Like any flu vaccine, there is the possibility of a reaction in the form of headaches, nausea or a high temperature. More rarely there are complications such as convulsions or neurological disorders. And, like a good hypochondriac, I find myself taking my temperature several times a day, checking the site of the injection for swelling or redness and wondering how symptoms might first make their appearance.

But I feel fine. Apart from a small headache, which I suspect was caused by spending too long at the computer screen, I've not had even a twinge of pain. So far I seem as healthy as ever.

Why would anyone put themselves on a clinical trial, knowing that every new therapy carries risks? I did it simply because I think, as many others do, that a flu pandemic is the greatest threat to human health that we face in the next few years, and that a vaccine is the only realistic chance we have of preventing very large numbers of deaths.

The Oxford Vaccine Group, led by Dr Andy Pollard, is carrying out the study into a prototype vaccine which is made from an inactivated form of the strain of bird flu currently circulating in south-east Asia. The first part of the trial involved testing the vaccine in different doses on 300 volunteers in Paris and the results of two of the doses were particularly promising. The second stage is now concerned with looking at the different responses in humans to those two doses - one of 7.5 micrograms, the other of 30 micrograms with an adjuvant, a chemical which enhances the immune response.

I am one of 150 volunteers in Britain who, along with around 450 volunteers in Belgium, have been put into one or other of the two groups, and will be given two more doses over the next year. In three weeks I will go for further blood tests and the serum in my blood will be analysed to see what kind of antibody response has been achieved. This is how doctors can measure the kind of defence the body can mount against an invader.

The beauty of a vaccine is that it allows doctors to give you some immunity against a disease which would otherwise be perilous. The principles that Edward Jenner first discovered when tackling cowpox - that you can 'teach' the body to respond in the right way - still hold true today. With the bird flu trial, doctors want to see whether you could produce a sufficient antibody response with a lower dose because with a limited supply of vaccine you could make it go much further and immunise many more people. Of course, there are risks. People can have bad reactions to vaccines, and in rare cases they can cause long-term damage. But vaccinations have enabled entire populations to remain free of disease and the last few years have seen big leaps in our ability to produce good vaccines. In the next few years, we could see a vaccine appear for malaria, which affects 10 per cent of the world's population.

Bird flu may seem a distant threat but the production of an effective vaccine is essential. We are used to the seasonal influenza strains which normally come around in winter so we produce antibodies against them, but we have no natural immunity at all to the one which has caused such panic, known as H5N1. A bird flu virus is a master of disguise and continually changes the proteins on its outer coat to prevent your immune system's memory cells from 'remembering' them and sending out antibodies.

If the H5N1 virus changes from being one which just infects birds and ducks to one which is capable of transmitting easily between people to become a world flu pandemic, our bodies could not mount any kind of successful defence against it. For those with chronic diseases or other immune problems, the risks would be very great.

The antiviral drug Tamiflu is being stockpiled in vast quantities by many countries, but the problem is that the capsules have to be taken as soon as symptoms begin, which will inevitably lead to huge pressures on GP services. It also remains unclear how effective it would be in a pandemic. Studies in animals suggest it would be good enough to keep most of us out of hospital, but until the pandemic strain emerges we cannot know its true efficacy.

If, however, scientists could create a vaccine which gave you even 30 per cent immunity against pandemic flu, then it would have a major impact on the course of the disease, preventing millions of potential deaths and preventing millions more from becoming infected, because it would limit and slow the spread. Experiments on animals at St Jude's Children's Hospital in Memphis, Tennessee, show that, while previous exposure to similar strains may not stop them falling ill when infected with H5N1 flu, it can prevent them dying.

An H5 vaccine that was not perfect could still prime people's immune systems, so that they would need only one shot of a specific pandemic vaccine.

There are lots of prototype vaccines being developed around the world, and in Britain the government has already ordered 3.5 million doses of two particular kinds. These are based on the existing strains of H5N1 and would not be a perfect match once the virus changed to become 'humanised', but it would be good enough to give the front-line doctors and nurses some immunity during a pandemic, when up to 25 per cent of the population would be infected.

The Department of Health has also said it will produce 'advance purchase agreements' for 120 million doses of vaccine when it does mutate into a pandemic form. This is to give manufacturers some investment now for expanding their capacity, so that when the mutation happens they can guarantee that we would have the right, exact-match vaccine ready within three to four months.

But what the world really needs is a new way of making vaccines. Traditionally, fertilised hens' eggs are used, but it is laborious and time-consuming. Switching to cell cultures would make more sense, and has worked for polio vaccine, but no one knows yet whether it will be possible for influenza. The entire vaccine capacity for all of Europe is capable of making 293 million doses of flu vaccine a year, for a population of 450 million.

The Oxford trial is not only monitoring side effects among the volunteers, but trying to work out what an effective dose might be. I don't know whether this vaccine would give me any more protection than anyone else, simply because we don't know how the virus will shift its shape over time.

But as Pollard said: 'This is a bit like a dress rehearsal. We have to start developing vaccines now so that we can answer some of the questions over dosage. In that way, if all the processes are in place, we can make a vaccine quickly if a pandemic strain emerges. This is an important study, on a pressing issue for human health.'